This invention relates to an extrusion molding machine for producing multilayer parisons used for blow molding which consists of two or more kinds of thermoplastic synthetic resins. More particularly, the invention relates to an extrusion molding machine for producing multilayer parisons which consists of a plurality of thermally joined layers of synthetic resins, where, by using said multilayer parisons for blow molding operation, multilayer molded articles having desired characteristics of respective constituent materials can be produced.
Up to now, the blow molded articles have been widely used as the containers for foodstuffs, medicines, cosmetics, chemical materials, detergents and so forth, however, they have been generally single layer products. As the raw materials for the blow molded articles, thermoplastic resins such as polyethylene, polypropylene and polyvinyl chloride are commonly employed.
In recent years, the saving of oil and electric resources, the disposal of waste matter such as plastic articles, and the harmful effect of plastic in view of food sanitation especially in connection with polyvinyl chloride products have become social problems. Therefore the above containers or bottles must also be produced with minimum possible material and, at the same time, they must satisfy the requirements in use and have no problem in view of environmental pollution and food sanitation. Nevertheless the ordinary monolayer blow molding products are not able to satisfy all the above-mentioned requirements simultaneously.
Taking the laminated film wrappings into consideration, the inventors of the present application have carried out eager and extensive studies on the introduction of laminated structure to the blow molded articles, and thereby the present invention has been accomplished. That is, several properties of thermoplastic resins such as hardness, flexibility, tensile strength, gas permiability, chemical resistance and printability are different with the kinds of resins. While the outer surface of a bottle is liable to receive various external effects said influences and the appearance of the bottle is mainly influenced by the kind of material of the outer surface. Further, when the bottle is to be printed, the outer material may preferably have affinity to printing inks. On the other hand, the inside surface of the plastic bottle comes into direct contact with the material contained therein, so that the inside surface of the bottle must neither react with the contents nor release any extraneous substance into the contents. Still further, there is no such requirement with regard to the intermediate layer, therefore the materials for the intermediate layer may be those having the additional properties which are insufficient in the above-mentioned outer and inner layers, if any, or those improving the properties of the whole layers.
In view of the above, when the plastic bottles are made double layer structure or triple layer structure and most suitable materials are selected for the respective layers, more desirable bottles can be produced from smaller amount of resin materials as compared with the conventional single layer bottles.